Adding a New Field to the Movie Model and Table

Note: An updated version of this tutorial is available
here that uses
ASP.NET MVC 5 and Visual Studio 2013. It's more secure, much simpler to
follow and demonstrates more features.

In this section you'll use Entity Framework Code First Migrations to
migrate some changes to the model classes so the change is applied to the
database.

By default, when you use Entity Framework Code First to automatically create
a database, as you did earlier in this tutorial, Code First adds a table to the
database to help track whether the schema of the database is in sync with the
model classes it was generated from. If they aren't in sync, the Entity
Framework throws an error. This makes it easier to track down issues at
development time that you might otherwise only find (by obscure errors) at run
time.

Setting up Code First Migrations for Model Changes

If you are using Visual Studio 2012, double click the Movies.mdf file from
Solution Explorer to open the database tool. Visual Studio Express for Web will show Database Explorer, Visual Studio 2012 will show Server Explorer. If you are
using Visual Studio 2010, use SQL Server Object Explorer.

In the database tool (Database Explorer, Server Explorer or SQL Server
Object Explorer), right click on MovieDBContext and select Delete
to drop the movies database.

Navigate back to Solution Explorer. Right click on the Movies.mdf file and select Delete
to remove the movies database.

Build the application to make sure there are no errors.

From the Tools menu, click Library Package Manager
and then Package Manager Console.

Right click on the red squiggly line under Movie and select
Resolve then usingMvcMovie.Models;

Doing so adds the following using statement:

using MvcMovie.Models;

Code First Migrations calls the Seed
method after every migration (that is, calling update-database
in the Package Manager Console), and this method updates rows that have
already been inserted, or inserts them if they don't exist yet.

Press CTRL-SHIFT-B to build the project. (The following
steps will fail if your don't build at this point.)

The next step is to create a DbMigration class for the initial
migration. This migration to creates a new database, that's why you deleted the
movie.mdf file in a previous step.

In the Package Manager Console window, enter the command
"add-migration Initial" to create the initial migration. The name
"Initial" is arbitrary and is used to name the migration file created.

Code First Migrations creates another class file in the Migrations
folder (with the name {DateStamp}_Initial.cs ), and this class contains code that creates the database schema.
The migration filename is pre-fixed with a timestamp to help with ordering.
Examine the {DateStamp}_Initial.cs file, it contains the
instructions to create the Movies table for the Movie DB. When you update the
database in the instructions below, this {DateStamp}_Initial.cs
file will run and create the the DB schema. Then the Seed
method will run to populate the DB with test data.

In the Package Manager Console, enter the command
"update-database" to create the database and run the Seed
method.

If you get an error that indicates a table already exists and can't be
created, it is probably because you ran the application after you deleted the
database and before you executed update-database. In that case,
delete the Movies.mdf file again and retry the update-database
command. If you still get an error, delete the migrations folder and contents
then start with the instructions at the top of this page (that is delete the
Movies.mdf file then proceed to Enable-Migrations).

Run the application and navigate to the /Movies URL. The seed
data is displayed.

Adding a Rating Property to the Movie Model

Start by adding a new Rating property to the existing
Movie class. Open the Models\Movie.cs file and add the Rating
property like this one:

Build the application using the Build > Build
Movie menu command or by pressing CTRL-SHIFT-B.

Now that you've updated the Model class, you also need to update
the \Views\Movies\Index.cshtml and \Views\Movies\Create.cshtml
view templates in order to display the new Rating property in the
browser view.

Open the \Views\Movies\Index.cshtml file and add a
<th>Rating</th> column heading just after the Price
column. Then add a <td> column near the end of the template to
render the @item.Rating value. Below is what the updated
Index.cshtml view template looks like:

You've now updated the application code to support the new Rating
property.

Now run the application and navigate to the /Movies URL. When you do
this, though, you'll see one of the following errors:

You're seeing this error because the updated Movie model class
in the application is now different than the schema of the Movie
table of the existing database. (There's no Rating column in the
database table.)

There are a few approaches to resolving the error:

Have the Entity Framework automatically drop and re-create the database
based on the new model class schema. This approach is very convenient when
doing active development on a test database; it allows you to quickly evolve
the model and database schema together. The downside, though, is that you
lose existing data in the database — so you don't want to use this
approach on a production database! Using an initializer to automatically
seed a database with test data is often a productive way to develope an
application. For more information on Entity Framework database initializers,
see Tom Dykstra's
ASP.NET MVC/Entity Framework tutorial.

Explicitly modify the schema of the existing database so that it matches
the model classes. The advantage of this approach is that you keep your
data. You can make this change either manually or by creating a database
change script.

Use Code First Migrations to update the database schema.

For this tutorial, we'll use Code First Migrations.

Update the Seed method so that it provides a value for the new column. Open
Migrations\Configuration.cs file and add a Rating field to each Movie object.

Build the solution, and then open the Package Manager Console
window and enter the following command:

add-migration AddRatingMig

The add-migration command tells the migration framework to
examine the current movie model with the current movie DB schema and create the
necessary code to migrate the DB to the new model. The AddRatingMig is arbitrary
and is used to name the migration file. It's helpful to use a meaningful name
for the migration step.

When this command finishes, Visual Studio opens the class file that defines
the new DbMIgration derived class, and in the Up
method you can see the code that creates the new column.

Build the solution, and then enter the "update-database" command in the
Package Manager Console window.

The following image shows the output in the Package Manager Console
window (The date stamp prepending AddRatingMig will be different.)

Re-run the application and navigate to the /Movies URL. You can see the new
Rating field.

Click the Create New link to add a new movie. Note that you
can add a rating.

Click Create. The new movie, including the rating, now shows
up in the movies listing:

You should also add the Rating field to the Edit, Details and
SearchIndex view templates.

You could enter the "update-database" command in the
Package Manager Console window again and no changes would be
made, because the schema matches the model.

In this section you saw how you can modify model objects and keep the
database in sync with the changes. You also learned a way to populate a newly
created database with sample data so you can try out scenarios. Next, let's look
at how you can add richer validation logic to the model classes and enable some
business rules to be enforced.

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